Apparatus for soft-nitriding



United States Patent 3,401,066 APPARATUS FOR SOFT-NITRIDING Robert Grafmiller, Southfield, Mich., assignor to Motor City Metal Treating Co., Detroit, Mich., a corporation of Michigan No Drawing. Filed Mar. 3, 1966, Ser. No. 531,369 4 Claims. (Cl. 14815.5)

ABSTRACT OF THE DISCLOSURE A process of nitriding a ferrous metal which includes immersing the metal in a salt bath containing both cyanate and cyanide, with the cyanate content of the bath being in excess of 40 percent. The bath is contained in a pot fabricated from a high chromium steel and the nitriding process is carried out at a temperature between 500 and 600 C.

The present invention relates to the soft-nitriding process, and more specifically to the composition of the pot in which this process is carried out. Sift-nitriding is a process for improving the fatigue and wear resistance of ferrous metals, especially low carbon and low alloy ferrous metals. It is carried out in fused salt baths having substantial amounts of cyanate and cyanide in the bath. It is a relatively low temperature process being generally carried out at temperatures between 500 to 600 C. The cyanate salt may be of any alkali metal, but its quantity is usually calculated as the potassium salt. The cyanide, too, may be of any alkaline metal, but it is usually calculated as the sodium salt. Soft-nitriding has been the subject of considerable literature especially in Germany. For example, it has been described in Germany by Dr. Johannes Muller in the September 1954 issue of Das Industrieblatt, Stuttgart, on pages 366 through 370 in an article entitled Nitriding Steel in the Salt Bath. In addition to the natural aeration of the salt bath from its exposed surface, soft-nitriding is frequently carried out with additional air being pumped into the bottom of the bath. Considerable difliculty has been encountered in the construction and design of the pot in which the soft-nitriding process is carried out. An example of a pot which may be utilized for soft-nitriding is shown in U.S.P. No. 3,022,204, entitled Process For Nitriding Metals. Many of the original pots used in the soft-nitriding process were made of low carbon steel. However, since the soft-nitriding process is designed to nitride the outer portion of ferrous parts, it can be readily understood and appreciated that substantial amounts of the iron pot would become nitrided, thereby lowering the cyanate content of the bath as the cyanate is the active agent and, in addition, slowly destroying the pot. Pot distruction becomes especially serious when soft-nitriding is carried out using more than 40 percent cyanate.

Therefore, it is the principal object of this invention to discover a relatively inexpensive pot composition for use in the soft-nitride process.

It is a further object of this invention to discover a relatively inexpensive pot composition for use in the softnitriding process where in the cyanate content of the salt bath is above 40 percent.

Applicant has achieved the above and other desirable results by constructing a soft-nitriding pot from a highchromium steel. It has been found that such a high-chromium steel pot has little appreciable effect on the cyanate content of the salt bath, and in addition, it is not corroded by the molten salts. Furthermore, it has been found that regardless of the cyanate content of the bath, soft-nitriding carried out in a high-chromium steel pot achieves excellent results. This result is in direct contrast to the prior art teaching found in Patent No. 3,208,885, issued on Sept. 28, 1965, and entitled, Apparatus for Nitriding of Metals. In this patent, the patentee taught that soft-nitriding could not successfully be carried out in a pot containing substantial quantities of iron, especially if the cyanate content of the molten salt bath were greater than percent, and, therefore, the patentee taught that it was much preferred to construct the pot or a pot liner from either aluminum or titanium in order to carry out soft-nitriding with more than 40 percent cyanate. However, aluminum is quite unsatisfactory since it begins to melt at a temperature only slightly above the preferred operating temperature of the molten salt bath. Titanium is so expensive that generally the pots are constructed of low carbon steel and then a titanium liner is positioned in such pots so that the molten salt bath contacts only the titanium liner. However, when liners are used, they must be fitted quite accurately to the pot, or they have a tendency to buckle. Therefore, it is much preferred, if at all possible, that liners be avoided. Owing to the moderate cost of stainless steels, it is economical that the pot itself be constructed from a high-chromium steel. However, stainless steel liners may be utilized in conjunction with low carbon steel pots.

High chromium steels containing about 17 percent chrome are most preferred, although the chromium content may vary from about 14 percent to 27 percent. In addition, nickel chromium steels may also be used, the nickel being added to improve the high temperature properties of the alloy.

The embodiments of the invention in which an exelusive property or privilege is claimed are defined as follows:

1. A process of nitriding a ferrous metal which comprises immersing said metal in a molten salt bath containing cyanide and a cyanate, said salt bath being contained in a pot having a high chromium steel surface containing about 14 to 27 percent chromium in contact with said molten salt bath.

2. The process of claim 1 wherein said pot is entirely constructed from said high chromium steel.

3. The process of claim 2 wherein said nitriding is carried out at a temperature between 500 and 600 C.

4. The process of claim 3 wherein said ferrous metal is low carbon steel.

References Cited UNITED STATES PATENTS 3,022,204 2/1962 Muller et a1 14815.5

OTHER REFERENCES Molten Salt Baths, Du Pont de Nemours & Co., Inc., 1942, relied on p. 51.

CHARLES N.-LOVELL, Primary Examiner.

U.S. DEPARTMENT OF COMMERCE PATENT OFFICE Washington, D.C. 20231 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,401,066 September 10, 1968 Robert Grafmiller It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 44, after "a cy anate" insert content in excess of 40% Signed and sealed this 30th day of December 1969.

( E Attest:

Edward M. Fletcher, Jr. WILLIAM E. SCHUYLER, JR.

Attesting Officer Commissioner of Patents 

